1. Field of the Invention
The present invention relates to an electric power steering apparatus of a type which is capable of generating an assisting force for a steering operation by means of an electric motor.
2. Description of the Prior Art
The above-described type of power steering apparatus generally includes speed reduction means provided between an electric motor and a steering shaft, and clutch means for disengaging the steering shaft from the motor at the time of the latter's failure, or the like. Such a power steering apparatus is disclosed in, for example, the specification of Japanese Patent Laid-Open No. 16958/1983.
The power steering apparatus of the type disclosed in the above specification comprises: a steering shaft; means for detecting the operation of the power steering apparatus; a motor driven in response to the output of the detection means for generating an assisting force for a steering operation; and speed reduction means including an input shaft driven by the motor, an output shaft connected to the steering shaft, and a planetary gear mechanism for reducing the rotational speed of the input shaft and transmitting the reduced rotational speed thereof to the output shaft. The planetary gear mechanism includes a sun gear which is rotatable together with the input shaft, a ring gear, planet gears connected to the output shaft such that the revolving motion thereof around the sun gear is transmitted to the output shaft, and a gear case for accommodating the sun gear, the ring gear, and the planet gears. The above-described apparatus further comprises an electromagnetic clutch means actuated in response to the output of the detection means to be switched over between a first position at which it locks the ring gear so that it cannot be rotated and a second position at which it releases the ring gear to allow it to be rotated with respect to the output shaft.
When the electromagnetic clutch means in the above-described power steering apparatus is at the first position, since the ring gear is locked so that it cannot be rotated, the planet gears revolve around the sun gear, and this revolving motion is transmitted to the output shaft. As a result, the rotation of the input shaft driven by the motor is transmitted through the planetary gear mechanism to the output shaft, so that an assisting force for the steering operation is applied to the steering shaft. When the detection means detects a failure of the motor or the like, the electromagnetic clutch means is switched over to the second position. This makes the ring gear rotatable, and revolution of the planet gears around the sun gear is thereby prevented. As a result, the input shaft is disengaged from the output shaft, and the steering operation in which the steering shaft is rotated by rotation of the steering wheel is enabled independently of the input shaft and the electric motor, eliminating the influence of the failed motor on the steering operation. The electromagnetic clutch means of this known power steering apparatus includes an electromagnetic solenoid provided on the outside of the gear case, and an operation pin protruding into the gear case, the operation pin being extended and drawn by the electromagnetic solenoid. In this electromagnetic clutch means, when the operation pin is extended, it engaged with the ring gear accommodated in the gear case, locking it so that it cannot be rotated. When the operation pin is drawn into the solenoid, the locking of the ring gear is released, and the ring gear becomes rotatable. This arrangement, however, is disadvantageous in that installation of the electromagnetic solenoid in a vehicle is troublesome because it protrudes from the gear case. Further, it has been necessary to use an electromagnetic solenoid of large size which generates a strong magnetic field in order to ensure that the ring gear is locked by this operation pin. It is also difficult to provide the operation pin with sufficient strength to ensure a good quality operation thereof.